Nanocrystalline explosives offer a number of advantages in comparison to conventional energetics including reduced sensitivity and improved mechanical properties. In this study, formulations consisting of 90% hexanitro-hexaazaisowurtzitane (CL-20) or cyclotrimethylene trinitramine (RDX) and 10% polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 200nm to 2m. The process to create these materials used a combination of aqueous mechanical crystal size reduction and spray drying. The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Compressive stress-strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall-Petch mechanics. In the most extreme case (involving CL-20/PVOH formulations), crystal size reduction from 2m to 300nm improved compressive strength and Young's modulus by 126% and 61%, respectively. These results serve to highlight the relevance of structure-property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nanoscale dimensions.